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Advice? How much river detail to show?

Hi all,

so I'm working on a map of a large island/mini-continent set in the middle of my world's largest inland sea. I'm having a bit of difficulty figuring out what level of detail to use in depicting the rivers though. There are a handful of large rivers feeding into the sea, and several smaller rivers feeding some large lakes. Should I show all of them? Or is the map too crowded?

The important ones to show would I think be (on the landmass at left) the river at upper left, the one below it, the one at bottom left, and (on the continent) the one at top, the one to the left of the mountains, the bottom horizontal river, and the one with the oxbow lake. I'm uncertain about the others - do I need to show the water source for every lake?

The attached map is just a VERY basic sketch, copied from my working map, which has a bit more detail but is all in layers at the moment. For scale, the island-continent is about 12 degrees north-to-south.

I usually depict any river that is important to the purpose of the map. Rivers that influence travel and navigation should be shown—if it has significant riverboat travel or is too wide and/or deep to be forded near most arbitrary locations. If it has a large impact on settlements in the area, then it should probably be shown. If it defines a political border, then it should probably be shown. A map of the physical geography of the area will probably show more rivers than a political map.

It may also be wise to keep several sheets or layers of rivers with different levels of detail in a master document so you can quickly generate different kinds of maps with the level of detail you need.

You don't necessarily have to show input to every lake. What I would expect is for more of those lake basins to have outlets.

Sometimes it's sufficient to show navigable rivers, or also those broad enough to be serious barriers. But a dense net of watercourses could also be used to hint this area I've here is wetter than that river-scarce area over there. Too, to a practiced eye a river network says as much about the relief as do explicit topography symbols or contours.

In fact, your network may be saying things you don't intend. For instance, for a river to parallel a coast means there's higher ground in a coastal strip, keeping water from otherwise draining off the land quicker. And when a river runs a LONG way near the coast, think what that implies about how high the coastal barrier must be, far upstream (since upstream = uphill).

Over the ages, all but the deepest and driest basins would tend to fill with water, then their overflow would cut the low spot even lower, until impenetrable bedrock was reached, or the climate dried enough to make evaporation overrule rainfall. The bigger the watershed, the less likely there wouldn't be enough rain to EvEnTuAllY fill it in. Sure, there'll be some DeadSea/ GreatSaltLake situations. But there's also US Great Lakes and African Great Rift Valley chains of lakes. When I use Fractal Terrains to generate landmasses, there are always too many dead-end basins. FT lets you do the other likely real-world eventuality - even if there's not huge blind basins full-to-brim of water, over ages some of those would fill with sediment, making for nice plains, steppes, and such , with maybe room for only small lakes , whether dead-end or overflowing.

Apparently, since even a brand- new continent but recently arisen from the depths or volcanically piled up or magically emplaced has to have rainwater runoff paths, you can figure some rivers predate the mountains - and cut channels and outlets AS the mountains are pushed up. You can see those cases all over a world map. So some of those FT basins I fill with dirt, and others I cheat and tweak outlet channels into place to drain. Wilbur can do some such alterations on an initial terrain set, I've just never learned it.

All that to say - don't just scatter rivers to drain all the land - a good first aim! But also figure your general highs and lows, and where they would push and pull the water. You don't have to devise a full set of ice ages and plate tectonics and climate shifts - but a few generalities will go a long way toward heightening plausibility.

To directly answer your question, once I was satisfied with river placement, for lots of map types I would keep at least as many as you show. I might slim them down, and/or mute their color a bit. Is your original map file big enough to permit several different widths of watercourses, and /or tapering them?

I would recommend finding a map that you like (or an area here on Earth that's about the same size) and seeing what others would show for it. Ultimately, as Midgardsormr pointed out, you should show the rivers that are important to why the map exists! Historically, maps were done for a purpose. Keep in mind why the map is being made when you work on it.

Also, as jbgibson pointed out, some of your rivers are doing things that require special terrain conditions. If you look at a contour map of the world (one with lines at differing heights), rivers almost always cross contour lines at right angles. If you sketch out the rough contours (mountains here, highlands here, hilly here, and so on), you'll find that it will help enormously in placing plausible rivers. It might even help you with how many rivers to show! I did a quick example that shows the opinions of a particular piece of software that moves things downhill:
You can force rivers to go like that, but the system would really prefer to run rivers more perpendicular to the coast. That map is a bit bigger than Borneo; check how rivers look on maps of that area (keeping in mind that Borneo is a very wet place)

Thanks for the good tips - particularly the point about the northern river. It looks obvious in hindsight, lol. This is one of those times I wish I could afford decent terrain software. In response to the question about tapering the rivers: yes, they're that way on my working map, or rather will be once I figure out which ones I'm keeping. Didn't want to waste my work on the small ones.

At this point the planet hasn't been settled yet (it's for a science-fantasy story) so no civilization to guide me. But I may end up taking out some of the smaller streams as I rework things anyway. I'll post more as I rework things.